In gas turbine applications, a single layer of partially yttria stabilized zirconia (YSZ) is typically deposited by electron beam-physical vapor deposition (EB-PVD) or air plasma spray (APS) techniques onto a metallic bond coating or substrate to act as a ceramic thermal barrier coating. When the YSZ is applied by EB-PVD, a columnar grained microstructure is obtained, where the columns, with minute spaces between them are oriented substantially perpendicular to the surface of the metallic bond coating which covers the metal substrate, as is well known in the art. Between individual columns of the EB-PVD coating are micron spaced gaps extending from the outer surface of the YSZ layer to within a few micrometers of the bond coating or its associated alumina layer, as described in U.S. Pat. No. 5,562,998 (Strangman). This ZrO.sub.2 columnar structure is also described in U.S. Pat. No. 4,321,311 (Strangman) where such columnar layer is about 100-175 micrometers (0.004-0.007 inch) thick, and where the cracks or gap openings between individual columns are in the sub-micron to one micron range.
The structure of the YSZ evolves during service at high temperatures. Sintering and/or transformation of the crystal structure leads to failure during thermal-elastic cycling. Loss of and/or segregation of yttrium at high temperatures can cause destabilization of the cubic-YSZ and tetragonal-YSZ structures. As a result, the ceramic structure transforms to monoclinic-zirconia on cooling. The volume change that results from the transformation to monoclinic-zirconia leads to spallation of the ceramic coating. Because of the ceramic failure mechanisms, it is critical to limit the maximum service temperature of the thermal barrier ceramic coating. The temperature of the thermal barrier ceramic (e.g., YSZ) can be limited by controlling the temperatures of the combustion process. However, increased turbine efficiency requires increased combustion temperatures.
Besides high temperature mechanical properties, excellent phase/thermal stability and high thermal expansion coefficients are desirable for thermal barrier coatings. Padture et al., in J. Am. Ceram. Soc, "Low Thermal Conductivity in Garnets", 80[4] 118-120, (1997) have suggested that polycrystalline garnets may be very useful in this regard, in advanced thermal barrier coatings, as a complete substitution for stabilized zirconia.
There is however still a need for new systems and methods of controlling temperatures in thermal barrier coating systems. The present invention is directed to these, as well as other, important ends and it is one of the main objects of this invention to provide such thermal barrier coating systems.